1. Field of the Invention
The invention relates generally to solar energy collector systems and pertains, more specifically, to a rim-drive cable-aligned heliostat collector system.
2. Description of the Prior Art
New developments in the solar thermal power station art include a recent trend to design and produce an economically viable alternative to conventional commercial electrical power generating facilities. To become a viable alternative, power stations must be capable of producing electrical power in the megawatt range. From this recent trend, central-receiver tower-power systems have emerged as one of the leading alternatives among the different solar power systems for generating electricity in the megawatt range.
To provide power in the megawatt range, central-receiver tower systems usually include an array of hundreds or even thousands of individually steered flat reflectors or mirrors and a central receiver on top of a tower. Such arrays are normally referred to as a heliostat field, while the individual reflectors are normally referred to as heliostats. The heliostat field redirects radiant solar energy to the receiver. The receiver collects and converts the radiant solar energy to thermal energy which is then normally used to drive a turbine generator for electrical power generation.
It will be appreciated that the structure of most heliostats fields is fairly complex. Thus, to fully understand the novel aspects of the present invention and the numerous advantages flowing therefrom, the general structure of the individual heliostats will necessarily briefly be described. Each individual heliostat generally includes a reflective panel, a foundation, trenching, and a two-axes tracking system. The reflector panel generally includes a reflector and a lattice-type metal reflector support frame by which the reflector is mounted. The reflector is usually in the form of a mirror module which comprises a plurality of laminated silvered-type mirrors.
The reflector panel is usually supported by a metal constructed tubular-shaped pedestal which elevates the heliostat to a desired vertical distance from the land surface. The foundation is usually formed from steel reinforced concrete and is employed to anchor the pedestal to the ground through the trenching.
The two-axes tracking system provides each heliostat with the proper orientation and normally includes a heliostat array controller that interfaces with a heliostat controller and a dual drive/motor sensor system. Both controllers usually comprise microprocessor-based units.
The heliostat array controller usually responds to commands from the heliostat controller and sends information to the heliostat controller. It also calculates commands for giving each heliostat its orientation through the drive/motor sensor system. Preprogrammed algorithms typically are used to affect orientation. Heliostat electronics convey signals between the data control network and the various heliostats.
Although tower power systems as aforedescribed have presently emerged as a viable alternative for commercially generating electrical power in the megawatt range, several major problems and concerns include firstly, the high cost associated with producing the heliostat field; secondly, the high cost associated with producing many of the components of the individual heliostats; thirdly, degredation and failure of the reflector panel due to weathering; and lastly, cleaning and maintenance techniques.
The concerns and problems centering around high production costs exist because, generally speaking, the cost of producing a heliostat field accounts for about 50%-60% of the total investment cost of the tower power system. This high cost for producing heliostat fields is in part due to the need to provide very accurate, complex and expensive two-axes tracking components on the hundreds of heliostats comprising the field. For example, in nearly all tower systems almost every single heliostat in the field requires at least two fairly expensive drive motors, one for rotating the heliostat reflector panel about the elevational axis and the other for rotating the heliostat reflector panel about the azimuthal axis.
The aforesaid high production cost is also in part due to the need to provide all of the aforesaid numerous heliostats with steel pedestals and reinforced steel concrete foundations. Generally speaking, these foundations and pedestals frequently account for as much as 13% of the cost to produce each individual heliostat. From the aforegoing, it can thus be seen that there is a need to lower the total investment cost of tower power systems in order to improve the attractiveness of tower systems as an alternative source of electrical power generation. Significantly, this need can be met by devising a two-axis tracking system, wherein the high cost associated with the two drive/motor components and the pedestal and foundation components is substantially reduced.
The concerns centering around the reflector panel exist because heliostats should have an operational life of around 30 years to be effective for commercial electrical power generation. Unfortunately, however, most panels are continuously exposed to adverse weathering, such as for example, sandstorms, hailstorms, wind storms, dust storms, lightening, and ice/snow load for the entire operating life thereof. Such weathering frequently prematurely destroys the heliostats by causing either substantial degradation of the reflective surface of the reflectors or by causing corrosion and failure of the reflector frames.
Degradation of the reflective surface, for example, usually results in optical reflectivity/specularity losses. Generally speaking, reflectivity is associated with the reflector material and the spectral variation in the absorption of radiant energy. Specularity is associated with the degree of scattering in light rays with respect to the reflector surface finish in flatness. Specularity is particularly significant in heliostat applications which rely on redirecting the radiant energy over long distances with minimal scattering. Consequently, when degradation causes the reflectivity and specularity to fall below desired values, the reflector must be either repaired or discarded and replaced.
Specifically speaking, substantial degradation of the reflector surface which leads to replacement of the heliostat occurs in many ways. For example, when the silvered reflector surfaces are exposed to precipitates of moisture such as salt and hydrogen sulfide, the precipitates frequently act adversely to irreversibly impair its optical reflectivity/specularity properties.
In another example, many steel reflector frames under atmospheric exposure produce a thin adherent surface layer of iron oxide as a corrosion product. When rain water washes over the oxidized steel surfaces, wets adjacent reflector surfaces, and subsequently dries thereon by evaporation, the iron oxide residue thereof stains the reflector surface. Such stains may also adversely affect reflectivity. Additionally, in those instances where it is still possible to clean the stained surface, the stains normally can only be removed through timely abrasive scrubbing or acid dissolution cleaning techniques.
Similarly, wind born particles such as sand often abraid the reflector surfaces or may even be deposited thereon. Such abrasion and deposits also adversely act to reduce reflectivity. Moreover, in those instances where it is still possible to clean the surfaces, such deposits often become difficult-to-remove contaminants or a source of unwanted staining.
With respect to failure of the reflector frame, ice/snow loads and wind loads on both the reflector and the reflector frame may cause fracture thereof, which may eventually lead to failure. Similarly, the impact of hail and other wind blown particles on the reflector and the reflector frame may also cause fracture thereof and eventual failure. Additionally, extreme temperature changes frequently cause permanent deformation and warping of the reflector frames, which again may also lead to fracture and eventual failure thereof. The concerns regarding degradation and failure of the reflector panel become even more acute when it is remembered that in large power systems hundreds of heliostats usually are involved.
Reflector cleaning methods are an area of concern in tower systems because numerous heliostats must be routinely cleaned, as well as repaired, to maintain heliostat field efficiency. However, the task of cleaning these heliostats is normally rather time consuming. In fact, generally speaking, the major cost of cleaning heliostats is directly related to the cleaning task time rather than to the method of cleaning. Thus, there is a current need to reduce the task time to an acceptable level in order to make the cleaning technique cost-effective on a commercial scale of operations.
To cope with the aforesaid problems and concerns, the two-axes tracing systems of some heliostats have been designed to integrate the components of the tracking system so as to reduce the number of components and thus minimize related material and labor costs. For example, considerable progress has been made in reducing the number of heliostat controllers and associated cable runs. Unfortunately, however, these integrated component-type designs as well as other prior art designs suffer from one or more disadvantages and shortcomings. For instance, in the aforesaid designs, nearly all of the tracking systems still require two fairly expensive drive/motor sensor components for each heliostat.
Some existing designs allow for the reflector panel to be rotated in an inverted or horizontal position, whereby the reflector surface is caused to face the ground to protect it from sand, hail and other harmful windblown particles. However, in these designs, the reflector panels are subject to deformation, fracture and eventual failure due to dead weight loads and ice/snow loads acting on the upward-facing frame side thereof.
In an attempt to prolong the operating life of the heliostats, some heliostat designs use fairly low-cost plastic materials to provide barrier coats and base coats to protect the reflectors from harmful precipitates and the like. However, in those cases where the heliostat reflectors are in the form of mold-in laminated-silvered or aluminized-films with molded or extruded plastic substrates, the process used to produce such reflectors is usually costly and often presents problems with adhesion. Additionally, the metallized-films of these reflectors often do not perform well. Moreover, in those cases where the heliostat reflectors are in the form of a silvered or aluminized-film laminated to a metallic or nonmetallic facing sheet, the surface finish of the molded laminate is often inadequate as a specular substrate.
Some heliostat designs have employed additional metal constructed stiffeners to strengthen the reflector frames in order to enable the reflector panels to more easily withstand the combined effects of wind, temperature and gravity loads. However, such stiffeners frequently increase the dead weight load of the entire panel, thereby also frequently increasing the potential for warping, fracture and eventual failure of the panel.
Some cleaning and maintenance procedures utilize a spray-soak technique to clean the heliostat. In this technique, a first truck is employed to spray a soak, clean and rinse-type of wash solution on the heliostat and a second truck is employed to spray-rinse the soaked heliostat. Unfortunately, however, the task of driving these trucks through a large heliostat field and cleaning hundreds of reflectors therein is repetitive to the point of boredom. As a consequence, inattentiveness of maintenance men during cleaning often leads to accidental damage to the heliostats with the trucks.
In an attempt to prevent such accidental damage, some cleaning and maintenance procedures employ mechanical scrubbers to clean the heliostats. However, this latter procedure requires the use of numerous scrubbing machines and a position and steering system for locating these machines near the heliostat.
One unique solution to the aforesaid problems utilizes a plurality of solar collectors in a rim-drive tracking system. The rim-drive tracking system provides an apparatus for suspending a plurality of solar collectors from a plurality of cables. Suspending the collectors from cables allows the number of required pedestal and foundation components to be reduced in a manner somewhat similar to that taught in applicant's present application hereinafter. This earlier rim-drive type of tracking system is described in U.S. Ser. No. 192,799, filed Oct. 1, 1980 and assigned to the same assignee as the present application, and which is incorporated herein by reference.
Notably, however, this earlier rim-drive tracking system does not describe a stowage technique for satisfactorily protecting the heliostats from weathering as is disclosed herein. It also does not disclose a heliostat frame which functions to substantially reduce heliostat stress during steering, nor does it describe a motor/drive system for steering as described hereinafter.